Technically you are right, ofcourse. An adware is unwanted software designed to show you ads maliciously (no way to remove or turn it off). But let's look at some common features of adware / spyware:
1. Advertisements appear in places they shouldn’t be. (Yes in Firefox - ads in new tab and address bar).
2. Your web browser’s homepage has mysteriously changed without your permission. (Yes in Firefox - default custom home page)
3. New toolbars, extensions, or plugins suddenly populate your browser. (Yes in Firefox - bundles unwanted, uninstallable extensions)
4. Your computer starts automatically installing unwanted software applications. (Yes in Firefox - studies can install extensions without your knowledge).
5. Collect personal data without your knowledge (Yes in Firefox - ad partners and studies).
Again, technically you are right that there are options to disable some of these things, (most of which are all enabled by default that the majority of users won't be aware of) ... But when a software imitates and behaves like an adware / spyware, Mozilla would do best to listen to their users criticism than call us ignorant.